Cubic boron nitride clusters

ABSTRACT

A cubic boron nitride cluster comprises a core ( 10 ) and an overgrown region, the overgrown region containing a plurality of cubic boron nitride crystallites ( 12 ) extending outwards from the core ( 10 ). The majority of the cubic boron nitride crystallites ( 12 ) have a cross-sectional area which increases as the distance from the core ( 10 ) increases. A method of producing cubic boron nitride clusters is also provided.

BACKGROUND OF THE INVENTION

This invention relates to the growth of cubic boron nitride (CBN)crystals as clusters of crystals.

The use of seeds to control crystallisation by controlling the number ofnucleation sites is well known in the art of crystal growing. In thecase of cubic boron nitride synthesis, small cubic boron nitrideparticles may be used as seeds to promote the preference of crystalgrowth on the seeds rather than crystal growth by spontaneousnucleation. For such applications, it is desirable to ensure that theseeds have a known size distribution, so that numbers of seeds can becontrolled, and that the seeds are distributed evenly and discretely.

Generally, in the art of growing cubic boron nitride crystals by highpressure, high temperature (HPHT) synthesis, the seeds may be cubicboron nitride particles which are single crystals selected on the basisof size alone. Such seeds are usually made by crushing larger cubicboron nitride crystals or may be as-grown cubic boron nitride crystals,and the cubic boron nitride crystals grown using these seeds aredominated overwhelmingly by single crystals. A method of growing cubicboron nitride crystals uses the difference in solubility betweenhexagonal boron nitride and cubic boron nitride under the sameconditions of pressure and temperature as the driving force forcrystallisation.

This method is otherwise known as the allotropic change method. Othermethods of generating supersaturation are known in the art. Generally,the objective of such cubic boron nitride growth is to maximise theproportion of individual and discrete crystals, and to minimise theproportion of clusters containing a plurality of crystals. Suchclusters, when they occur, may be due to secondary nucleation on thesurface of growing crystals, or may be due to the accidental proximityof two or more seeds and/or growing crystals.

SUMMARY OF THE INVENTION

According to the present invention, a method of growing cubic boronnitride clusters includes the steps of providing a source of boron andnitrogen, providing a plurality of growth centre particles, eachcomprising a bonded mass of constituent particles, producing a reactionmass by bringing the source of boron and nitrogen and growth centreparticles into contact with a crystallisation agent, subjecting thereaction mass to conditions of elevated temperature and pressuresuitable for crystal growth, allowing sufficient time for crystal growthto occur and recovering the cubic boron nitride crystal clusters fromthe reaction mass.

The growth centre particle will provide a number of nucleation siteswhich may be randomly oriented by virtue of its structure and theinitial crystals that grow will exhibit a variety of crystallographicdirections depending upon the growth centre's structure. Some of thesecrystals will be oriented so that they grow in the fastest growingdirection, whilst other crystals will grow more slowly. Depending uponthe number of nucleation sites in the growth centre, the degree ofinterference of adjacent growing crystals and their growth directions,the growth of some crystals will be terminated early whilst others willcontinue growing. This will result in a crystal cluster whose structureis related to the structure of the original growth centre. Furthermore,when the constituent particles comprising the growth centre particlehave twin planes, the resultant grown crystal cluster will comprisecrystallographically twinned crystals. Moreover, the twinning structureof the growth centre particle may contribute to faster growth inparticular crystallographic directions and so play a role in theselection of terminated crystals and those that continue to grow.

Thus, it has been found that the method of the invention producesclusters of cubic boron nitride crystals, with the number of crystalscomprising the cluster ranging from a few crystals to several hundredcrystals, or more. The cubic boron nitride cluster comprises a core andan overgrown region, the overgrown region containing a plurality ofcubic boron nitride crystallites extending outwards from the core, themajority of the crystallites having a cross-sectional area whichincreases as the distance of the crystallite from the core increases.Such clusters are believed to be new and form another aspect of theinvention. The crystallites of the clusters are generally substantiallyfaceted and substantially free of crystallisation agent. Such clustersmay be made up of predominantly single crystals, or predominantlytwinned crystals. Generally, at least 80% of the crystallites have across-sectional area which increases as the distance of the crystallitefrom the core increases.

It is possible by appropriate selection of the growth centre particlesto produce clusters of selected and controlled or tailored structure,including clusters with significant aspect ratios.

The clusters may be used in abrasive particle applications such asgrinding, sawing, cutting, turning, milling, drilling, boring orpolishing. Clusters will have better bond retention and/or free cuttingproperties than other conventional cubic boron nitride abrasives.

Claddings or coatings may be applied to clusters to further enhancetheir utility in particular applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a secondary micrograph of a fractured cubic boron nitridecluster at a magnification of about ×135;

FIG. 1 b is a view of the same fractured cubic boron nitride cluster asin FIG. 1 a seen as a mixed secondary electron and cathodoluminescenceimage at a magnification of about ×135. The picture shows the growthcentre to the centre left of the cluster; and

FIG. 2 is a secondary electron micrograph (magnification of about ×135)of the same cluster as shown in FIG. 1 viewed from the reverse side.

DESCRIPTION OF EMBODIMENTS

The source of boron and nitrogen may be a single source or a mixture ofsources. In the case of a single source of boron and nitrogen, thesource may be any non-cubic form of boron nitride such as hexagonalboron nitride or any other boron nitride known in the art of cubic boronnitride synthesis. In the case of a mixture of sources of boron andnitrogen, the source may be a mixture of the aforesaid sources, amixture of magnesium boride and magnesium nitride, a mixture of othersuitable nitrides and borides, or a combination of these or any othersuitable source of boron or nitrogen.

The growth centres may be derived from CVD cubic boron nitride, shockwave produced cubic boron nitride, HPHT cubic boron nitride, orpolycrystalline cubic boron nitride (PCBN). Growth centre particlesbeing a bonded mass of constituent cubic boron nitride particles providea multiplicity of nucleation sites, the number of which are controlledby the selection of a suitable combination of constituent particle sizerange and growth centre size range. The constituent particles of thegrowth centre may be randomly oriented crystallographically. Theconstituent particles may be of any suitable size, but typically, willhave a size of less than 200 microns. The growth centre particles may beof any size, but typically, will have a size less than 1 millimeter.

The bonding in the growth centre particles is such as to create arelationship, generally a predetermined relationship, between individualconstituent particles. The bonding may be self-bonding betweenconstituent particles or by means of a bonding agent which may beorganic or inorganic.

Growth centre particles from CVD cubic boron nitride may be provided bycrushing CVD film and screening to a suitable size range. The crushedCVD particles may be agglomerated to form the growth centre using asuitable binder. The size of the growth centre may be controlled by asuitable sizing technique, such as sieving. The constituent CVDparticles contain a plurality of twin planes, the average spatialdensity of which depends upon the nature of the CVD film from which theyare derived. Hence, the growth centre particle will contain amultiplicity of constituent particles with a multiplicity of twinplanes.

Growth centres from HPHT cubic boron nitride may be provided byselecting a suitable size fraction of cubic boron nitride particles,granulating the cubic boron nitride using a suitable binder, andproducing a suitable size range of growth centre particles by a suitablesizing technique, such as sieving.

Growth centre particles of polycrystalline cubic boron nitride (PCBN),may be provided by selecting a PCBN of suitable grain size, and crushingto a suitable size range. Growth centre particles of this type willcontain a multiplicity of constituent particles (grains), a proportionof which may be microtwinned.

Growth centre particles may have any aspect ratio, i.e. length to widthratio.

Crystallisation agents for the synthesis of cubic boron nitride are wellknown in the art. Examples of such crystallisation agents are alkalimetal elements such as lithium and alloys containing these elements, andthe nitrides and boron nitrides of these elements. Other suitablecrystallisation agents for the synthesis of cubic boron nitride arealkaline earth elements, such as calcium and magnesium, alloys ofalkaline earth elements, and the nitrides and boron nitrides of theseelements. Further examples of suitable crystallisation agents are alkaliand alkaline earth fluoronitrides, water, hydrogen chloride, borazine,hydrazine, boranes and a selection of organic compounds.

The source of boron and nitrogen and the growth centre particles arebrought into contact with the crystallisation agent to create a reactionmass. Generally, the source of boron and nitrogen and the growth centreparticles will be mixed with the crystallisation agent in particulateform. There must be sufficient boron and nitrogen source to create asupersaturation in the crystallisation agent or react to form anyintermediate compound and provide for growth of the cubic boron nitridecrystal clusters to the desired size.

Crystallisation and crystal structure modifiers may be introduced intothe reaction mass to achieve specific objectives, such as modifying theelectrical, semiconducting and mechanical properties and stoichiometryof the grown crystals.

The reaction mass may be placed in a reaction capsule which is placed inthe reaction zone of a high temperature/high pressure apparatus and thecontents then subjected to the desired elevated conditions oftemperature and pressure. The source of boron and nitrogen reacts,dissolves or decomposes and the species migrate to the surface of thegrowth centre particle and precipitate or grow thereon. The constituentcrystals of the resultant cubic boron nitride clusters will have amorphology and predominance of single crystals or crystallographic twinsdepending on the saturation-time profile utilised, as well as thetemperature and pressure conditions, the chemical composition of thecrystallisation agent, and the crystallographic structure of theconstituent particles of the growth centre particles.

The conditions of elevated temperature and pressure which are used inthe method may be those under which cubic boron nitride isthermodynamically stable. These conditions are well known in the art.Generally, the elevated temperature will be in the range 1200 to 2200°C., and the elevated pressure will be in the range 4 to 8 GPa. Theseconditions of elevated temperature and elevated pressure are maintainedfor sufficient time to allow the cubic boron nitride cluster to grow tothe desired size. The time will be generally greater than 10 minutes andcan be several hours.

It is also possible to produce cubic boron nitride growth underconditions which are outside the region of thermodynamic stability ofcubic boron nitride. Conditions of temperature and pressure outside theregion of thermodynamic stability of cubic boron nitride can be used ifthe Ostwald rule dominates the growth process rather than theOstwald-Volmer rule (see S Bohr, R Haubner and B Lux Diamond and Relatedmaterials volume 4, pages 714–719, 1995)—“According to the Ostwald rule,if energy is withdrawn from a system with several energy states, thesystem will not reach the stable ground state directly, but instead willgradually pass through all intermediate stages. In addition, accordingto the Ostwald-Volmer rule, the less dense phase is formed (nucleated)first. Where the two rules would appear to contradict each other, theOstwald-Volmer rule has priority over the Ostwald rule.” In the case ofcubic boron nitride crystal growth outside its region of thermodynamicstability, the Ostwald-Volmer rule can be suppressed by, for example,the application of pressure, thus allowing the growth of cubic boronnitride on pre-existing cubic boron nitride particles, providedhexagonal boron nitride crystals are substantially absent.

Isothermal and isobaric conditions are preferred in the method of thisinvention. However, other methods of generating conditions for cubicboron nitride crystal growth such as the temperature gradient method andsize dependent supersaturation, may be used.

Recovery of the cubic boron nitride clusters from the reaction mass maybe carried out by methods well known in the art, e.g. by hydrolysing thecrystallisation agent using water, or any other suitable methoddepending upon the crystallisation agent used.

The invention will be illustrated by the following examples.

EXAMPLE 1

A mixture was made of lithium boron nitride and hexagonal boron nitride,and a small quantity of self-bonded cubic boron nitride particles, 44 to74 microns. The mixture was compacted into a cylinder by isostaticcompaction and machined to a size so that it fitted into the reactioncapsule of a high pressure high temperature apparatus. The reactioncapsule was raised to conditions of about 1500° C. and 5.1 GPa. Theseconditions were maintained for a period of 35 minutes. The reactioncapsule was returned to normal pressure and temperature and the contentsof the reaction mass removed from the reaction capsule. The reactionmass was hydrolysed with hot water and then fused with sodium hydroxide.The crystals recovered after this treatment were in the form of clustersas shown in FIGS. 1 and 2. The clusters were amber in colour, averagedabout 350 microns in overall diameter and comprised 20 to 40 constituentcrystals each up to about 200 microns across. The clusters comprised acore 10 and an overgrowth region 12—see FIG. 1 b. The overgrown region12 consists of a number of crystallites which have a cross-sectionalarea which increases as the distance of the crystallite from the coreincreases.

EXAMPLE 2

Growth centre particles were made by compacting a mass of cubic boronnitride powder with an average particle size of 3 microns, and crushingand grading the compact to provide particles with a size range of 45 to75 microns. A quantity of these growth centre particles was mixed withhexagonal boron nitride and a lithium nitride as crystallisation agent.The mixture was then heat treated in the manner described in Example 1.The recovered cubic boron nitride clusters were generally of the formshown in FIGS. 1 and 2.

EXAMPLE 3

A quantity of the growth centre particles of Example 2 were mixed with a9:1 mixture of hexagonal boron nitride and magnesium. The mixture wascompacted and shaped to form a reaction mass, fitted into a reactioncapsule and raised to conditions of about 1350° C. and about 4,4 GPa.These conditions were maintained for about 30 minutes. After recovery,the cubic boron nitride clusters were found to be of the same generalform shown in FIGS. 1 and 2. The clusters had an average size of about100 microns and comprised 15 to 30 crystals at the cluster surface.

1. A cubic boron nitride cluster comprising a core and an overgrownregion containing a plurality of cubic boron nitride crystallitesextending outwards from the core, the majority of the crystalliteshaving a cross-sectional area which increases as the distance of thecrystallite from the core increases.
 2. A cubic boron nitride clusteraccording to claim 1 wherein at least 80% of the crystallites have across-sectional area which increases as the distance of the crystallitefrom the core increases.
 3. A cubic boron nitride cluster according toclaim 1 which is substantially free of a crystallisation agent for thesynthesis of cubic boron nitride.
 4. A cubic boron nitride clusteraccording to claim 1 wherein the crystallites are substantially faceted.5. A cubic boron nitride cluster according to claim 1 wherein the corecomprises a bonded mass of constituent cubic boron nitride particles. 6.A method of producing a plurality of cubic boron nitride clustersincludes the steps of providing a source of boron and nitrogen,providing a plurality of growth centre particles, each growth centreparticle comprising a bonded mass of constituent particles, producing areaction mass by bridging the source of boron and nitrogen and growthcentre particles into contact with a crystallisation agent, subjectingthe reaction mass to conditions of elevated temperature and pressuresuitable for crystal growth, allowing sufficient time for crystal growthto occur and recovering the cubic boron nitride crystal clusters fromthe reaction mass.
 7. A method according to claim 6 wherein the sourceof boron and nitrogen is hexagonal boron nitride, other non-cubic formof boron nitride, a mixture of two or more such boron nitrides or amixture of a nitride and a boride.
 8. A method according to claim 6wherein the constituent particles of the growth centre particles arecubic boron nitride.
 9. A method according to claim 8 wherein the cubicboron nitride for the growth centre particles is selected from the groupconsisting of CVD cubic boron nitride, shock wave produced cubic boronnitride, HPHT cubic boron nitride, polycrystalline cubic boron nitrideand a combination of two or more thereof.
 10. A method according toclaim 6 wherein the constituent particles have a size of less than 200microns.
 11. A method according to claim 6 wherein the growth centreparticles have a size of less than 1 mm.
 12. A method according to claim6 wherein the bonding in the growth centre particles is achieved byself-bonding between constituent particles.
 13. A method according toclaim 6 wherein bonding between constituent particles in the growthcentre particles is achieved by means of a bonding agent.
 14. A methodaccording to claim 6 wherein the elevated temperature is in the range1200 to 2200° C. and the elevated pressure is in the range 4 to 8 GPa.